This concerns a disposable electrode and a method for its manufacture.
Electrodes are used in numerous medical applications either for monitoring or stimulation or for both. For example, they are used to pickup bioelectric signals in electrocardiography and electroencephalography. They are used for electrical stimulation in pain control devices and in defibrillators. Whatever their application, it is necessary that the electrodes provide for good electrical contact with the patient's skin and good electrical signal transmission between the patient and electrical leads to the monitoring or from the stimulation devices.
Typically, a bioelectric electrode comprises a lead wire leading to or from an electrical apparatus and an interface medium between this lead wire and the patient's skin. The interfacing medium is the crucial element in the electrode, since it has to answer a rigid set of requirements. Such requirements are that the medium be a good conductor, be physically conforming to the skin surface, adhere strongly to skin, and retain these characteristics over a considerable period of time.
Typically, the medium used in most electrodes is a conductive semiliquid gel which can be made according to large numbers of formulations, such as those set forth in U.S. Pat. No. 3,027,333 to Friedman, and in U.S. Pat. No. 3,567,657 to Lichtenstein, or other formulas available from numerous commercial sources. A semiliquid gel has several disadvantages, the most serious is that it is messy and tends to seep out from a pad in which it is impregnated (such a pad is described in U.S. Pat. No. 3,998,215 to Anderson and Gumbusky) or from the open cells of a polymeric sponge in which the gel is trapped such as described in U.S. Pat. No. 3,901,218 to Buchalter in U.S. Pat. No. 3,989,035 to Zuchisdorff and in U.S. Pat. No. 4,027,664 to Heavner et al.
The instability of the semiliquid gel led to the invention of other alternatives for an interfacing medium which could eliminate the messiness and seepage of the gel. For example, a self drying conductive collodium was proposed by Adolph in U.S. Pat. No. 3,607,788. A more solid gel, having a consistency of Mayonaise was proposed by Buchalter in U.S. Pat. No. 3,989,050.
Still other solutions involve the use of rigid conductive gels, which could be self supporting, i.e., could be used as a sheet of material placed between the conductive wire and the skin. Such material made of high consistency gel, resembling rubber pads, can conform to the skin surface, and their material does not spill or disintegrate. Moreover, some rigid gels have the additional characteristic of tackiness, i.e., they adhere well to the skin and eliminate, totally or partially, the need for adhesive elements which are used conventionally around the gel-wire combination in an attempt to provide firm contact. The need for this is apparent because in many cases the electrode has to stay for long periods on the skin (as in electrocardiographic monitoring) or stay on the skin when the patient moves vigourously (as in electrocardiographic ergometry). Tacky, rigid gels have been proposed by Berg (U.S. Pat. No. 4,066,078), who devised a gel made of carboxylic esters and sulphated cellulose esters, plasticised by glycerol. Another alternative is that proposed by Hymes (U.S. Pat. No. 4,125,110), where a combination of a naturally occuring polysaccharide, namely Karaya, and hydric alcohol were used to form a sheet of rigid gelatinous material.
The teaching of the last two patents cited are that the end result is satisfactory as long as one considers the tackiness and the conformity characteristics. However, the conductivity is not always the desirous one for electrocardiographic monitoring, as in the case of the Karaya which is referred to in the Hymes patent.
Furthermore, the preparation of electrodes made from these type of rigid gel is a relatively complex and lengthy process, when compared to the electrode preparation using liquid or semiliquid gel. In the last case, the gel is simply poured into a cavity or onto a nonwoven material used to trap the gel. In the case of the rigid sheets of gels, the gel has to be processed first into large sheets, cured and dried, and only then it is cut into smaller pieces to fit the lead wire, or lead pad. Since a very important characteristic of disposable electrodes is their low cost, improvement in the gel preparation can be a considerable advantage.